1. Field of the Invention:
The present invention relates to an amplifier circuit and, in particular, the present invention relates to an electronic amplifier circuit with a transistor and a transformer.
2. Description of the Related Art:
In modern mobile telecommunication, LNAs (low-noise amplifiers) are employed in the front end of a receiver. Such LNAs should generally meet several demands. First, they are supposed to have a low noise number and an amplification prescribed by the customer (mostly between 15 dB and 18 dB). Further demands would for example be low current consumption at a highest possible compression point P1dB and high third-order intermodulation IP3, so as to be able to achieve high sensitivity in the receiver. In order to provide a low noise number, for this purpose most of the time a single-stage emitter basic circuit of npn bipolar transistors (npn-BJTs) is employed. Such npn bipolar transistors in emitter basic circuit, however, often do not have sufficient amplification when taking all parameters into account in the design of a LNA.
In order to increase the amplification of such LNAs, often multi-stage amplifiers are used. But thereby the noise number is degraded by some tenths of dB, so that this partly takes on unacceptably high values. Furthermore, significantly higher current consumption than in LNAs with a single-stage emitter basic circuit of an npn bipolar transistor can be registered. Moreover, the third-order intermodulation IP3 and the compression point P1dB achievable by such multi-stage amplifiers are often influenced negatively when the amplification is increased. In order to compensate for these disadvantages, one possibility is to make the npn bipolar transistor (npn-BJT) smaller. This method, however, has similar problems as they occur in multi-stage amplifiers.